The present invention relates to a ceramic honeycomb filter for removing particulates from an exhaust gas from diesel engines.
To remove particulates from an exhaust gas emitted from diesel engines, investigation has been made to use ceramic honeycomb filters having porous partition walls through which the exhaust gas containing particulates is caused to flow. Such filters are called particulates-capturing filters (diesel particulate filters). FIG. 2(a) is a front view showing a ceramic honeycomb filter 11 as a particulates-capturing filter, FIG. 2(b) is a partially cross-sectional side view showing the ceramic honeycomb filter 11 of FIG. 2(a), and FIG. 3 is a schematic cross-sectional view showing the ceramic honeycomb filter 11 of FIG. 2. As shown in FIGS. 2 and 3, a substantially cylindrical ceramic honeycomb filter 11 comprises an outer peripheral wall 11a, and porous partition walls 11b disposed inside the outer peripheral wall 11a, with flow paths 11c surrounded by the outer peripheral wall 11a and the porous partition walls 11b or by the adjacent porous partition walls 11b sealed alternately by sealers 12a, 12b at inlet-side ends 11d and outlet-side ends 11e. The outer peripheral wall 11a of the ceramic honeycomb filter 11 supported by holding members 13a, 13b are received in a metal housing 14.
An exhaust gas containing particulates flows into the flow paths 11c of the ceramic honeycomb filter 11 through the inlet-side ends 11d (shown by 10a), passes through the porous partition walls 11b, and goes out of the adjacent flow paths 11c through the outlet-side ends 11e (shown by 10b). In this process, particulates contained in the exhaust gas are captured by pores in the porous partition walls 11b. When the captured particulates are excessively accumulated in the ceramic honeycomb filter 11, the pressure loss of the filter 11 increases, likely resulting in decrease in an engine output. Accordingly, the captured particulates are periodically burned by an external igniting means such as an electric heater and a burner to regenerate the ceramic honeycomb filter 11. Generally employed, when such a ceramic honeycomb filter 11 is used as a particulates-capturing filter, are (a) an alternate regeneration method using a pair of ceramic honeycomb filters, in which one filter is used while the other filter is regenerated, (b) a-continuous regeneration method, in which particulates are burned by the action of a catalyst while capturing particulates, to regenerate the filter, etc.
Important to such a particulates-capturing filter are a pressure loss, particulates-capturing efficiency, and particulates-capturable time, a time period from the start of capturing particulates to a point at which a pressure loss reaches a predetermined level. The capturing efficiency and the pressure loss are in a contradictory relation; the higher capturing efficiency results in increase in the pressure loss, while decrease in the pressure loss results in the reduction of the capturing efficiency. To satisfy these contradictory properties of the filter, it was conventionally investigated to control the porosity and average pore diameter of the porous partition walls of the ceramic honeycomb filter. It is further necessary that particulates captured by the ceramic honeycomb filter can be burned at high efficiency, and that the ceramic honeycomb filter is not broken by thermal stress generated by the burning of particulates. Thus, investigation has been conducted to meet these requirements.
Under such circumstances, JP 3-10365 B discloses a filter for cleaning an exhaust gas with little pressure loss, the filter having partition walls whose pores are composed of small pores having a pore diameter of 5-40 xcexcm and large pores having a pore diameter of 40-100 xcexcm, the number of the small pores being 5-40 times that of the large pores, whereby high capturing efficiency can be maintained from the start. In this filter, pores in the partition walls preferably have an average pore diameter of more than 15 xcexcm, and the cumulative pore volume of the pores is preferably within a range of 0.3-0.7 cm3/g. Though JP 3-10365 B does not describe the porosity P (volume %) of the partition walls, assuming that cordierite used in Examples has a density p of 2.5 g/cm3, the porosity P of the partition walls can be calculated from the cumulative pore volume V (cm3/g) by the following formula:
P(%)=100 Vxcfx81/(11+Vxcfx81).
According to the above formula, a preferred range (0.3-0.7 cm3/g) of the cumulative pore volume of pores in the partition walls is converted to a porosity of 42.8-63.6% by volume.
JP 61-54750 B discloses that by adjusting an open porosity and an average pore diameter, it is possible to design a filter from a high-capturing rate to a low-capturing rate. JP 61-54750 B provides a preferred specific example of the open porosity and the average pore diameter in a region defined by points 1, 5, 6 and 4 in FIG. 8. The open porosity and the average pore diameter of each point are as shown below.
JP 9-77573 A discloses a honeycomb structure having a high capturing rate, a low pressure loss and a low thermal expansion ratio, which has a porosity of 55-80% and an average pore diameter of 2540 xcexcm, pores in its partition walls being composed of small pores having pore diameters of 5-40 xcexcm and large pores having pore diameters of 40-100 xcexcm and the number of small pores being 5-40 times that of large pores.
Though a good balance between the pressure loss and the particulates-capturing efficiency of a filter can be achieved to some extent by optimizing the porosity and the average pore diameter of a ceramic honeycomb structure and the pore diameters of its partition walls, however, increase in the porosity and the average pore diameter inevitably results in decrease in the strength of the porous partition walls of the filter. The reason therefor is that the strength of the porous partition walls is in a contradictory relation with the porosity and the average pore diameter of the porous partition walls. Particularly when the porosity is increased to 60% or more, or the average pore diameter is increased to 15 xcexcm or more to provide a filter with a low pressure loss, there is remarkable decrease in the strength of the porous partition walls. Accordingly, it has been impossible to obtain ceramic honeycomb filters having low pressure loss and high capturing efficiency as well as high durability, which are not broken by thermal stress and shock generated when used as particulates-capturing filters for diesel engines, or by mechanical stress generated by fastening in assembling and vibration, etc.
At the time of regenerating a conventional honeycomb filter in which particulates are captured, because an exhaust gas passes through flow paths adjacent to an outer peripheral wall 11a as shown in FIG. 3, heat generated by the combustion of the captured particulates dissipates to a metal housing via the outer peripheral wall 11a and holding members 13a, 13b. Accordingly, there is a large temperature gradient between a center portion of the filter 11 and an outer periphery portion, resulting in the problems that the filter is cracked by thermal stress, and that particulates are insufficiently burned because there is no enough temperature increase in the outer periphery portion.
Further, because sealers on the exhaust-gas-entering side have flat outer end surfaces in conventional ceramic honeycomb filters as shown in FIGS. 2 and 3, particulates are accumulated on the outer end surfaces of sealers on the exhaust-gas-entering side. In addition, because particulates extremely strongly tend to be aggregated, the accumulation of particulates gradually grows. When there is a large accumulation of particulates, flow paths are clogged on the exhaust-gas-entering side, resulting in increase in the pressure loss of the filter. As a result, the particulates-capturable time becomes shorter, thereby making it necessary to regenerate the filter frequently.
Accordingly, an object of the present invention is to provide a ceramic honeycomb filter having excellent mechanical strength and durability as well as low pressure loss, which ensures a long particulates-capturable time.
As the result of intensive research in view of the above object, the inventors have found that by providing a ceramic honeycomb structure with porous partition walls having a pore diameter distribution in a predetermined range, and by improving a sealing method, it is possible to obtain a ceramic honeycomb filter having low pressure loss, high capturing efficiency and high strength together with excellent regeneration efficiency and long particulates-capturable time. The present invention has been completed based on this finding.
Thus, the ceramic honeycomb filter of the present invention comprises a ceramic honeycomb structure having porous partition walls defining a plurality of flow paths for flowing an exhaust gas through the porous partition walls to remove particulates from the exhaust gas, the predetermined flow paths among the flow paths being sealed at their ends, the porous partition walls having a porosity of 60-75% and an average pore diameter of 15-25 xcexcm when measured according to a mercury penetration method, and the maximum of a slope Sn of a cumulative pore volume curve of the porous partition walls relative to a pore diameter obtained at an n-th measurement point being 0.7 or more, the Sn being represented by the following formula (1):
Sn=xe2x88x92(Vnxe2x88x92Vnxe2x88x921)/[log Dnxe2x88x92log (Dnxe2x88x921)]xe2x80x83xe2x80x83(1),
wherein Dn is a pore diameter (xcexcm) at an n-th measurement point, Dnxe2x88x921 is a pore diameter (xcexcm) at an (nxe2x88x921)-th measurement point, Vn is a cumulative pore volume (cm3/g) at an n-th measurement point, and Vnxe2x88x921 is a cumulative pore volume (cm3/g) at an (nxe2x88x921)-th measurement point.
The ceramic honeycomb filter preferably has the maximum Sn of 0.9 or more. The ceramic honeycomb filter preferably has a porosity of 65-70%. The ceramic honeycomb filter preferably has an average pore diameter of 18-22 xcexcm.
A porous ceramic forming the ceramic honeycomb structure preferably has a main component chemical composition substantially comprising 42-56% by mass of SiO2, 30-45% by mass of Al2O3, and 12-16% by mass of MgO, the main component of its crystal phase being cordierite.
FIG. 15xe2x80x2 is a graph showing the relation between the cumulative pore volume and pore diameter obtained by eleven measurements at pore diameters of from 1 xcexcm to 140 xcexcm in Example D2 of U.S. Pat. No. 6,541,407 to Beau et al.
It is preferable that the flow paths near its outer peripheral wall are preferably sealed by sealers at both ends, that the length of the sealers from a filter end surface is 8.2% or less of the total length of the filter, and that the flow paths having both ends sealed exist within a range of a radial length corresponding to 5 times the partition wall pitch at maximum from the outer periphery toward the center of the filter. To seal the ends of the flow paths by sealers, the honeycomb structure is immersed in a sealer slurry to a predetermined depth with the predetermined openings of the honeycomb structure at an end covered by a resin mask, and the resin mask is removed after drying the slurry, followed by sintering of the sealers.
It is preferable that a pitch of the porous partition walls is 2.54 mm or less, and that at least some of the sealers sealing flow paths except for those near its outer periphery wall project from the end surfaces of the partition walls by 0.01-5 mm in a flow path direction.
The ceramic honeycomb filter of the present invention comprises a ceramic honeycomb structure having porous partition walls defining a plurality of flow paths for flowing an exhaust gas through the porous partition walls to remove particulates from the exhaust gas, the predetermined flow paths among the flow paths being sealed at their ends, a catalyst being carried by the porous partition walls, the porous partition walls having a porosity of 60-75% and an average pore diameter of 15-25 xcexcm when measured according to a mercury penetration method, and the maximum of a slope Sn of a cumulative pore volume curve of the porous partition walls relative to a pore diameter obtained at an n-th measurement point being 0.7 or more, the Sn being represented by the following formula (1):
Sn=xe2x88x92(Vnxe2x88x92Vnxe2x88x921)/[log Dnxe2x88x92log (Dnxe2x88x921)]xe2x80x83xe2x80x83(1),
wherein Dn is a pore diameter (gm) at an n-th measurement point, Dnxe2x88x921 is a pore diameter (xcexcm) at an (nxe2x88x921)-th measurement point, Vn is a cumulative pore volume (cm3/g) at an n-th measurement point, and Vnxe2x88x921 is a cumulative pore volume (cm3/g) at an (nxe2x88x921)th measurement point.
The partition walls of the honeycomb structure having the above structure have a high porosity and a sharp pore diameter distribution, with a high percentage of pores near its average pore diameter. Accordingly, the ceramic honeycomb filter of the present invention is low in pressure loss and high in strength.